1. Field of the Invention
The present invention relates to an image information reading apparatus, and more particularly, to an apparatus for reading image information on sheets by utilizing reflected and transmitted light beams.
2. Description of the Related Art
Recently have been developed an image information reading apparatus, called a color scanner, which can read image information on a sheet, such as a document, photograph, etc. The reading apparatus is provided with a light source lamp for exposing the sheet, with the image information thereon, to light. A fluorescent lamp is used as the light source lamp on account of its low costs and capability of emitting white light which covers a wide range of wavelength. A light beam emitted from the fluorescent lamp is guided to a photoelectric converter after being reflected by or transmitted through the surface of the sheet bearing the image information. The light beam incident upon the converter is converted thereby into an electrical signal, which is transmitted. The transmitted signal is converted into an image signal by, for example, a host computer.
In conventional image information reading apparatuses, a scanner is used to read an image from a sheet (U.S. Pat. No. 4,704,638). The scanner, which is composed of a straight-tube fluorescent lamp and a photoelectric converter, such as a CCD line sensor, is mounted on a carriage substrate. On the carriage substrate, the fluorescent lamp and the photoelectric converter are arranged in parallel with each other, with respect to their longitudinal direction. Also, the lamp and the converter are opposed to the sheet surface in order that a light beam reflected by the sheet is incident upon the converter. The carriage substrate is reciprocated (for sub-scanning) at right angles to the longitudinal direction of the line sensor, along the sheet surface with the image information thereon, by conventional drive means. While the substrate is reciprocating in this manner, a light beam is emitted from the fluorescent lamp toward the sheet surface, and the light beam reflected by the sheet surface is read by the photoelectric converter. Thus, an image is read from the desired sheet by the scanner, including the fluorescent lamp and the photoelectric converter, is driven along the sheet surface for scanning.
In these image information reading apparatuses with the scanner, the read image information or the quality of the read image often proved uneven with respect to the longitudinal direction of the straight-tube fluorescent lamp and the line image sensor. The following factor has been found out as a cause of this awkward situation.
In the reading apparatuses, the scanner is housed in a chassis which is hermetically isolated from the outside air. The chassis also contains a power source circuit, various control circuits, devices for scanning, and other elements which inevitably produce heat. Therefore, the chassis is provided with some ventilation means for cooling these heat generating elements. Conventional cooling methods includes, for example, a method in which the heated elements are cooled forcibly by means of blowers. According to this method, the outside air is introduced into the chassis by the blowers, thereby cooling the inside of the chassis. As the open air flows into the chassis, the temperature distribution therein becomes uneven. If the temperature distribution inside the chassis is uneven with respect to the longitudinal direction of the photoelectric converter, for example, the tube-wall temperature of the fluorescent lamp, mounted on the carriage substrate, is also uneven with respect to the longitudinal direction. It is well-known that the luminous efficacy of a fluorescent lamp is highly susceptible to the ambient temperature. In general, the luminous efficacy is the highest when the ambient temperature ranges from 40.degree. C. to 50.degree. C. If the temperature is higher or lower, then the efficacy is lower. As the luminous efficacy lowers, the illuminance of light emitted from the lamp lowers correspondingly. If the tube-wall temperature of the lamp is uneven especially with respect to its longitudinal direction, therefore, the illuminance distribution of the lamp is also uneven with respect to the longitudinal direction. Thus, the quality of the image information read by the photoelectric converter is lowered.